1. Field of the Invention
The present invention relates to textile fabric materials having improved antistatic properties and more particularly to flexible bulk containers made from such fabric material adapted to suppress generation and dissipate static electricity.
2. Description of Related Art
Flexible bulk containers have been utilized for a number of years to transport and deliver finely divided solids such as cement, fertilizers, salt, sugar, and barite, among others. Such bulk containers can in fact be utilized for transporting almost any type of finely divided solid. The fabric from which they are constructed is a weave of a polyolefin, specifically polypropylene, which may or may not receive a coating of a similar polyolefin on one or both sides of the fabric. If such a coating is applied, the fabric will be non-porous, while fabric without such coating will be porous. The usual configuration of such flexible bulk containers involves a rectilinear or cylindrical body having a wall, base, cover and a closable spout secured to extend from the base or the cover or both.
Such containers are handled by placing the forks of forklift hoist means through loops attached to the container. The weight of such bulk container when loaded is usually between 500 pounds and 4,000 pounds, depending upon the density of the material being transported.
Crystalline (isotactic) polypropylene is a particularly useful material from which to fabricate monofilament, multifilament or flat tape yarns for use in the construction of such woven fabrics. In weaving fabrics of polypropylene, it is the practice to orient the yarns mono-axially, which may be of rectangular or circular cross-section. This is usually accomplished by hot-drawing, so as to irreversibly stretch the yarns and thereby orient their molecular structure. Fabrics of this construction are exceptionally strong and stable as well as being light-weight.
Examples of textile fabrics of the type described above and flexible bulk containers made using such fabrics are disclosed in U.S. Pat. Nos. 3,470,928, 4,207,937, 4,362,199, and 4,643,119, the disclosures of which are incorporated herein by reference.
It has been found that the shifting of specific materials within the bulk container as well as friction created between the material and the container during loading and unloading of the container creates localized pockets of built-up static electricity in the container. Spark discharges from the charged container can be dangerous in dusty atmospheres or in close proximity to inflammable solvents, and can be quite uncomfortable to workers handling such containers.
One proposed technique for dissipating electrostatic charges that might otherwise build up during the handling of bulk containers is to provide a fabric wherein conductive yarns are interwoven with the other yarns used in the weaving of the fabric. For example, Canadian Patent 1,143,673 discloses a fabric construction based on polyolefin yarn wherein conductive fibers such as carbon fibers are interwoven longitudinally with the polyolefin yarn and connected to conductive connecting means at the base of the container. This conductive connecting means is adapted to be grounded so that localized static electricity build up does not occur while the container is being filled or emptied.
U.S. Pat. No. 4,431,316 discloses a similar fabric construction comprising a laminate of a first layer of woven polymeric fabric, a second layer of woven polymeric fabric, and an intermediate layer positioned between said first and second woven layers comprising a polymeric material which acts as a moisture barrier. At least one of the woven layers contains spaced threads of staple metal fibers which are disclosed to provide a path in the fabric along which charged ions may travel and a convenient point for electric corona discharge where the conductive fibers protrude outwardly from the container.
One of the disadvantages of these types of construction is that the container made therefrom must be grounded during the fill and emptying operations to provide a path for electrical discharge. Failure to ground the container can lead to the same sort of static build up and the consequent hazard of spark discharge discussed above.